1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus. More particularly, the present invention relates to an image forming apparatus of the type including a plurality of image carriers, primary image transferring means for transferring images formed on the image carriers to an intermediate image transfer body one above the other to thereby form a composite image, and secondary image transferring means for transferring the composite image to a sheet or recording medium.
2. Description of the Background Art
Today, a color copier, color printer or similar color image forming apparatus is spreading and includes either a single photoconductive drum or a plurality of photoconductive drums arranged in a tandem configuration. In the color image forming apparatus including a single drum, a plurality of developing units are arranged around the drum, and each forms a toner image on the drum in a particular color. Toner images so formed on the drum are transferred to a sheet one above the other, completing a full-color image. In the tandem color image forming apparatus, toner images each are formed on one of the drums by a particular developing unit in a particular color and sequentially transferred to a sheet one above the other to form a full-color image.
The color image forming apparatus with a single drum is small size and low cost. However, to form a full-color image, the apparatus has to repeat image formation a plurality of times (usually four times) with the drum and is therefore not feasible for high-speed image formation. By contrast, the tandem image forming apparatus can form a full-color image with a plurality of (usually four) drums and therefore at high speed although it is bulky and high cost.
The tandem color image forming apparatus uses either one of a direct image transfer system and an indirect image transfer system. In the direct image transfer system, intermediate image transferring devices corresponding one-to-one to the drums transfer toner images of different colors from the drums to a sheet being conveyed by a conveying belt one above the other. In the indirect image transfer system, primary image transferring devices transfer toner images of different colors from the drums to an intermediate image transfer belt one above the other. Subsequently, a secondary image transferring device transfers the resulting full-color image from the intermediate image transfer belt to a sheet.
A problem with the direct image transfer system is that a sheet feeder and a fixing unit should be respectively positioned upstream and downstream of the plurality of drums arranged along the conveying belt, increasing the size of the apparatus body in the direction of sheet conveyance. By contrast, the indirect image transfer system allows the secondary image transfer devices to be relatively freely laid out, so that the sheet feeder and fixing unit can be arranged one above the other below the drums. This successfully reduces the overall size of the apparatus body.
Another problem with the direct image transfer system is that when the fixing unit is positioned near the most downstream drum in order to reduce the size in the direction of sheet conveyance, a sufficient path for a sheet to bend cannot be provided between the drum and the fixing unit. Consequently, the fixing unit is apt to adversely influence image formation effected at the upstream side due to an impact ascribable to the leading edge of a sheet entering the fixing unit or a difference between the speed of the sheet passing the fixing unit and the speed of the conveying belt. The indirect image transfer system guarantees a sufficient path for a sheet to bend and is therefore free from such a problem. For this reason, the tandem color image forming apparatus, particularly one using the indirect image transfer system, is attracting increasing attention.
However, in the indirect image transfer type, tandem color image forming apparatus, the surface of the intermediate image transfer body and the surfaces of the drums move in contact with each other at primary image transfer positions. The intermediate image transfer body and drums therefore wear little by little or their surface characteristics vary little by little. Although the above configuration is unavoidable for primary image transfer, even the drums other than the drum to be used must be held in contact with the intermediate image transfer body. For example, in a black mode, the drums other than the drum assigned to black must also be held in contact with the intermediate image transfer body. As a result, the life of such drums is shortened.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-352883 and 2001-296716.
It is an object of the present invention to provide an image forming apparatus capable of reducing the deterioration of image carriers and an intermediate image transfer body contacting each other to thereby extend their lives.
An image forming apparatus of the present invention includes a plurality of image carriers and a plurality of toner image forming devices each for forming a toner image of a particular color on a respective image carrier. A primary image transferring device includes an intermediate image transfer body capable of contacting the image carriers. Toner images formed on the image carriers are sequentially transferred to the intermediate image transfer body one above the other by primary image transfer at positions where the image carriers contact the intermediate image transfer body, completing a composite toner image. A secondary image transferring device transfers the composite toner image from the intermediate image transfer body to a sheet or recording medium by secondary image transfer. The apparatus is selectively operable in a first mode that uses all of the toner image forming devices or a second mode that uses at least one toner image forming device, but does not use at least one toner image forming device. In the second mode, the image carrier associated with the toner image forming device not joining in image formation is released from the surface of the intermediate image transfer body at least until the end of the primary image transfer.